IsotopicstudyonDokrianiBamakglacier,centralHimalaya: implicationsforclimaticchangesandicedynamics Vasant NIJAMPURKAR, 1 Kameswara RAO, 1 Manmohan SARIN, 1 Joseph GERGAN 2 1 Physical Research Laboratory, Navrangpura, Ahmedabad 380 009, India E-mail:sarin@pr1.ernet.in 2 Wadia Institute of Himalayan Geology, Dehradun 248 001, India ABSTRACT . Measurements of natural and artificial radioisotopes ( 32 Si, 210 Pb and 137 Cs) and oxygen isotopes (d 18 O) have been carried out on surface snow and ice, shallow snow pits and an ice core collected from Dokriani Bamak glacier, central Himalaya, to study the dynamics of glacier ice and short-term climatic changes. Based on the 32 Si and 210 Pb activities in the meltwaters, the age of the snout ice is 400years and the flow rate of icealongtheglacierlengthis  14 m a ^1 .Thespecificactivityof 137 Cs,correspondingto1963 fallout,inthesurfaceiceattheequilibriumlineyieldsaflowrateof32ma ^1 , a factor of two higher than that derived for the snout ice.The depth variation of 137 Cs concentration in a shallow ice core yields a mean accumulation rate of 0.43ma ^1 for the glacier ice over the past decade.The d 18 O of snout ice (^13.4%) is significantly depleted compared to the aver- age value of ^9.2% in the shallow ice core, indicating that cooler climatic conditions pre- vailedaround AD 1600. Basedonthe oxygen isotopic ratios inthe shallow pits, an``altitude effect''of 0.9% per100min d 18 O variation is documented for this glacier. 1.INTRODUCTION Glaciers and ice sheets play an important role in the under- standing of present and past environmental changes, as well as providing information on the areal deposition patterns of nuclear and volcanic events. Polar glaciers are known to pre- servetheserecordssystematicallyformanythousandsofyears due to the minimal melting of ice in Arctic and Antarctic regions (Delmas and others, 1982; Von Gunten and others, 1983; Jouzelandothers,1987). Incontrast, temperateglaciers (located in high altitudes) hold potential records of environ- mental changes covering time-scales of a few centuries (Thompsonandothers,1997). Inthis context, radioactiveiso- topes serve as excellent time markers, while stable isotopes are sensitive indicators ofclimatic change.The naturalradio- isotopes of both short and long half-lives ( 3 H  12.3years; 210 Pb  22.3years; 32 Si  178years (as revisedbyNijampurkar and others,1998); 14 C  5730years) and the artificial radio- isotope 137 Cs (half-life  30years) have been used to derive the accumulation rates and ages of young and older ice in polar andtemperate regions.The stable-isotope composition of snow (d 18 O and dD) deposited in the polar and mid-lati- tude regions depends mainly on the air temperature and sources of moisture (Nijampurkar and Bhandari,1984; Peel andothers,1988).The d 18 Oand dD measurements, therefore, canbeusedtounderstandthedepositionalhistoryofice,past climatic variations, sublimation and homogenization pro- cesses occurring inthe upper layers of glacier ice. Duringthepast30years,severalstudieshavebeencarried out in polar and temperate regions, particularly Antarctic, Arctic and Alpine regions, using radioactive and stableiso- topes (Dansgaardandothers,1969; Jouzelandothers,1987). Studies fromthe Indian and Nepal Himalaya are relatively sparse (Yasunari 1976; Grabczak and others, 1983; Nijam- purkar and Bhandari, 1984; Mayewski and others, 1986; Wake, 1989; Nijampurkar and Rao, 1993). Recent studies based on d 18 O in snow/ice and ice core from the Tibetan (Xizang) Himalaya have addressed the seasonal relation- ship between d 18 O in snow/ice and air temperature and moisture sources (Aizen and others,1996; Yao and others, 1996; Thompson and others, 1997). Yao and others (1996) demonstrated that the d 18 O^T relationship has a positive slope over the northern part of theTibetan Plateau and sug- gestthattheslope``inverts''overanarrowbandbetweenthe Himalaya andthe centralplateauto the north. TheclimateoftheglaciatedcentralHimalayanregionis influenced by precipitation during the summer monsoon which develops over the Asian countries. Over India, the monsoonal rains begin by mid-June. The southwest mon- soonalsoplaysanimportantroleduringtheablationperiod of Himalayan glaciers. Precipitation in the form of snow occurs only during October to March, when the primary source of moisture is related to the winter monsoon and western disturbances.The accumulation is maximumwhen the snowline recedes to about1500m in the western Hima- laya and to about 3000 m in the eastern Himalaya. The main objectives of this studyare: (i) to obtaininfor- mationontheice dynamics and snow-accumulation rates of Dokriani Bamak glacier, and (ii) to decipher the relation- ship, if any, between the annual mean d 18 O content in sur- face snow and elevation with respect to glacier orientation. Theisotopicdataonsnow/iceandshallowicecore,thuscol- lected, are evaluated interms of their implications for short- term climatic changes and the depositional history of the glacier ice. Journal of Glaciology , Vol. 48, No .160, 2002 81